Transistors used for most flat panel displays typified by a liquid crystal display device and a light-emitting display device are formed using silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon provided over glass substrates. Further, transistors formed using such silicon semiconductors are used in integrated circuits (ICs) and the like.
Attention has been directed to a technique in which, instead of the above silicon semiconductors, metal oxides exhibiting semiconductor characteristics are used for transistors. Note that in this specification, a metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor.
For example, a technique is disclosed in which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductor and the transistor is used as a switching element or the like of a pixel of a display device (see Patent Documents 1 and 2).
Further, a technique is disclosed in which in a transistor including an oxide semiconductor, a highly conductive oxide semiconductor containing nitrogen is provided as buffer layers between a source region and a source electrode and between a drain region and a drain electrode, and thereby the contact resistance between the oxide semiconductor and the source electrode and between the oxide semiconductor and the drain electrode is reduced (see Patent Document 3).
Further, a technique is disclosed by which in a top-gate transistor including an oxide semiconductor, a channel region, a source region, and a drain region are formed in a self-aligned manner (see Non-Patent Document 1).